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This chalky-white, crustose epiphyte forms extensive, thick, powdery patches towards the base of large, mature trees. Sometimes it can give the impression that the trees have been whitewashed. It is virtually unmistakable; it contains Trentepohlia as the algal partner, so will scratch golden with a fingernail. A dab of Potassium hydroxide (K) turns it pale-lemon yellow. It is relatively common in suitable habitats throughout Ireland.

Key characteristics

The massive, chalk-white stains on trees, golden scratch test, and K reaction will distinguish it